1. Field of the Invention
The present invention generally relates to an apparatus for detecting deterioration of a catalytic converter of an internal combustion engine while avoiding error by performing the decision as to deterioration of the catalytic converter in the state where the temperature thereof rises. More particularly, the invention is concerned with the catalyst deterioration detecting apparatus for an internal combustion engine which can detect deterioration of the catalytic converter with enhanced reliability and accuracy by detecting accurately the temperature-rise state of the catalyst and increasing the frequency of the chance for the catalyst deterioration decision.
2. Description of Related Art
Heretofore, in an internal combustion engine (hereinafter also referred to simply as the engine) of a motor vehicle, a catalytic converter has been employed for the purpose of eliminating harmful components such as HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxides) from the exhaust gas of the engine for purification thereof.
On the other hand, since the combustion efficiency of the engine changes in dependence on the air-fuel ratio of a mixture gas charged into the engine, a feedback control of the air-fuel ratio (A/F) has been adopted in order to control the air-fuel ratio so that it can assume a stoichiometrically optimal value (e.g. 14.7) which conforms to the operation state of the engine. To this end, an air-fuel ratio sensor such as an O2-sensor or the like is mounted in an exhaust pipe of the engine at a position upstream of the catalytic converter for realizing the air-fuel ratio feedback control.
In this conjunction, there has also been proposed such a dual-sensor type air-fuel ratio control system in which an additional air-fuel ratio sensor is additionally provided at a position downstream of the catalytic converter in order to protect the control performance of the system against degradation which may be brought about due to variance in the output characteristics among the air-fuel ratio sensors. A typical one of such dual-sensor type air-fuel ratio control systems is disclosed, for example, in U.S. Pat. No. 3,939,654.
In general, in the region of the exhaust pipe located downstream of the catalytic converter, the temperature of the exhaust gas is low and undergoes less change. Besides, the harmful components such as mentioned previously have been eliminated from the exhaust gas by the catalytic converter. Thus, the air-fuel ratio sensor mounted downstream of the catalytic converter is well protected against adverse influences. Additionally,it is noted that in the exhaust pipe region mentioned above, the exhaust gas has been mixed sufficiently, and thus the oxygen concentration is uniform.
Thus, the dual-sensor type air-fuel ratio control system such as mentioned above can realize the air-fuel ratio feedback control with high stability and accuracy, because the air-fuel ratio sensor disposed at the downstream side of the catalytic converter is stable in respect with the output characteristic thereof.
It is further noted that when the catalytic converter undergoes deterioration in the course of time lapse under the unfavorable conditions such as use of different fuels, exposure to unburned gas and the like, harmful exhaust gas will be discharged without being purified. Under the circumstances, such arrangement has also been adopted in which the states of the catalytic converter are detected on the basis of the output signals of the dual or paired air-fuel ratio sensors mentioned above, to thereby generate an alarm signal when deterioration of the catalytic converter is detected.
For better understanding of the present invention, the background techniques thereof will be reviewed below in some detail.
FIG. 9 is a functional block diagram showing schematically a basic arrangement of a hitherto known or conventional catalyst deterioration detecting apparatus for an internal combustion engine known heretofore which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 225203/1995 (JP-A-7-225203).
Referring to FIG. 9, an internal combustion engine (hereinafter referred to simply as the engine) 1 is provided with an exhaust pipe 15 for discharging an exhaust gas G from the engine 1 to the atmosphere. A catalytic converter 10 is installed in the exhaust pipe 15 for purifying on the whole the harmful components such as HC, CO and NOx contained in the exhaust gas G.
A first air-fuel ratio sensor 11 is mounted in the exhaust pipe at a position upstream of the catalytic converter 10 with a second air-fuel ratio sensor 12 being disposed downstream of the catalytic converter (hereinafter also referred to simply as the catalyst) 10, wherein the first and second air-fuel ratio sensors 11 and 12 output air-fuel ratio signals V1 and V2, respectively, which indicate the oxygen concentrations of the exhaust gas G prevailing at the locations of these sensors.
In FIG. 9, a catalyst activation decision means 101 makes decision on the basis of the engine operation state as to whether or not the catalyst 10 is activated (i.e., whether or not the temperature of the catalyst is sufficiently high). When it is decided that the catalyst 10 is activated, the catalyst activation decision means 101 issues an activation signal C.
Further provided is a catalyst deterioration decision means 102 which is designed to operate in response to the activation signal C for deciding on the basis of the air-fuel ratio signals V1 and V2 whether or not the catalyst 10 suffers deterioration.
Connected to the catalyst deterioration decision means 102 is an alarm means 19 which is actuated when deterioration of the catalyst is determined.
An air-fuel ratio control means 103 is adapted to perform the air-fuel ratio control for the engine 1 on the basis of the air-fuel ratio signals V1 and V2.
Next, description will be directed to the operation of the conventional air-fuel ratio control system shown in FIG. 9.
In the operating state of the engine 1, the harmful components such as mentioned previously are eliminated from the exhaust gas G discharged from the engine 1 by means of the catalytic converter 10. The air-flow ratio sensors 11 and 12 detect the oxygen concentrations of the exhaust gas G to output the air-fuel ratio signals V1 and V2, respectively, which assume different values in dependence on whether the air-fuel ratio of the exhaust gas G is lean or rich relative to the theoretical or stoichiometric air-fuel ratio.
The air-fuel ratio control means 103 performs the air-fuel ratio control for the engine 1 on the basis of the air-fuel ratio signals V1 and V2. On the other hand, the catalyst activation decision means 101 issues the activation signal C to the catalyst deterioration decision means 102 when it is decided that the catalyst 10 is in the activated state.
The catalyst deterioration decision means 102 operates only when the activation signal C is inputted, to thereby make decision as to the deterioration of the catalyst 10 on the basis of the air-fuel ratio signals V1 and V2. When deterioration of the catalyst is determined, the catalyst deterioration decision means 102 drives the alarm means 19 to issue an alarm signal.
FIG. 10 is a schematic block diagram showing a hardware arrangement of a conventional catalyst deterioration detecting apparatus for an internal combustion engine. In the figure, like parts or components as those described above by reference to FIG. 9 are denoted by like reference symbols, and repeated description thereof is omitted.
The engine 1 is equipped with an intake pipe 2 for supplying a mixture gas to the engine 1, wherein an air, cleaner 3 is disposed at a position close to an inlet port of the intake pipe 2 for the adsorbing and eliminating dusts and other particles carried by the air taken in. Further, an intake manifold 4 is formed at an interface portion between the engine 1 and the outlet port of the intake pipe 2.
In the engine system shown in FIG. 10, a fuel injector 5 for fuel injection into the engine is disposed within the intake pipe 2 at a position upstream of a throttle valve 7. It should however be appreciated that the fuel injector 5 may be disposed at a position downstream of the throttle valve 7. Of course, the fuel injector 5 may be provided in association with each of the individual engine cylinders.
A heated-wire type air-flow sensor 6 constitutes an engine operation state detecting means together with other various sensors described hereinafter and is adapted to detect an intake air quantity (intake air flow rate in g/sec) Qa supplied to the engine 1 from the intake pipe 2 through the intake manifold 4.
The throttle valve 7 mentioned above is installed within the intake pipe 2 at a position downstream of the fuel injector 5. Provided in association with the throttle valve 7 is a throttle sensor 8 which serves for detecting an opening degree of the throttle valve 7.
Implemented integrally with the throttle sensor 8 is an idle switch 9 which detects an idling operation state of the engine when the throttle valve 7 is fully closed, to thereby issue an idle signal A.
An ignition coil 13 which is constituted by a boosting transformer generates a high voltage for ignition in response to an ignition signal P supplied from an igniter 14 to thereby trigger the ignition of the gas mixture within the cylinder of the engine 1. As will be described hereinafter, the ignition signal P is supplied as a signal indicating rotation (rpm) of the engine 1 to an electronic control unit (hereinafter also referred to simply as the ECU in abbreviation) 100.
The igniter 14 is constituted by a power transistor serving for interrupting a current flowing through a primary winding of the ignition coil 13.
Further provided is a thermistor-type water temperature sensor 16 which serves for detecting a temperature Tw of water employed for cooling the engine 1.
Through manipulation of a key switch 17, power supply from an on-board battery 18 is started, whereupon operation of the ignition system is effectuated.
The alarm means 19 may be constituted, for example, by an alarm lamp which is driven or actuated upon every occurrence of various abnormal events.
Further provided is a vehicle speed sensor 20 which is designed for outputting as a vehicle speed signal Vs a pulse signal having a frequency which is in proportion to the rotation speed (rpm) of the axle of a motor vehicle on which the engine 1 is installed.
Provided in association with the catalyst 10 is a catalyst temperature sensor 22 for detecting the catalyst temperature Tc.
The ECU 100 which may be comprised of a microcomputer is designed to serve for the functions of the catalyst activation decision means 101, the catalyst deterioration decision means 102 and others for controlling the operations of the fuel injector 5, the alarm means 19 and others on the basis of the various sensor signals (i.e., engine operation state signals).
As the various sensor signals inputted to the ECU 100, there can be mentioned the intake-air quantity signal Qa generated by the air-flow sensor 6, the throttle opening degree signal xcex8 supplied from the throttle sensor 8, the idle signal A generated by the idle switch 9, the first air-fuel ratio signal V1 originating in the first air-fuel ratio sensor 11, the second air-fuel ratio signal V2 originating in the second air-fuel ratio sensor 12, the ignition signal P generated in response to the interruption of the primary current flowing through the ignition coil 13, the cooling water temperature signal Tw supplied from the water temperature sensor 16, the vehicle speed signal Vs supplied from the vehicle speed sensor 20, and the catalyst temperature signal Tc supplied from the catalyst temperature sensor 22.
The ECU 100 is supplied with electric power from the onboard battery 18 upon closing of the key switch 17 to generate a driving signal supplied to the ignitor 14 in response to not only the air-fuel ratio signals V1 and V2 but also the other signals indicative of the engine operation states.
Furthermore, the ECU 100 determines arithmetically the fuel injection quantity on the basis of the air-fuel ratio signals V1 and V2 and the other engine operation state signals to thereby perform the feedback control of the air-fuel ratio with the aid of the driving signal supplied to the fuel injector 5. Additionally, the ECU 100 issues the actuation signal to the alarm means 19 upon occurrence of abnormal event.
Next, referring to flow charts of FIGS. 11 and 13 together with a waveform diagram of FIG. 12, description will be made of the catalyst activation decision processing procedure in the conventional air-fuel ratio control apparatus of the engine system shown in FIGS. 9 and 10. Parenthetically, FIG. 11 is a flow chart for illustrating processing procedure executed by the catalyst activation decision means 101 incorporated in the ECU 100, and FIG. 13 is a flow chart for illustrating processing procedure executed by the catalyst deterioration decision means 102 incorporated in the ECU 100. In FIG. 12, reference voltages VR1 and VR2 are used for determining rich and lean air-fuel ratios respectively.
In the description which follows, it is assumed that the catalyst temperature sensor 22 as employed is an inexpensive sensor which can not ensure measurement of the catalyst temperature over a wide range and that the catalyst activation decision means 101 incorporated in the ECU 100 is designed to arithmetically determine or estimate the activated state of the catalytic converter 10 (catalyst temperature) on the basis of the load state of the engine 1.
To this end, the catalyst activation decision means 101 includes a counter for measuring a time lapse under predetermined load conditions. By way of example, the catalyst activation decision means 101 may be so designed as to increment a counter value CNT in response to the intake air quantity Qa which bears correspondence to the engine load.
Now referring to FIG. 11, the catalyst activation decision means 101 makes decision as to whether or not the current catalyst activation decision processing is initial after the turn-on or closing of the key switch 17 (step S901).
When it is decided in the step 901 that the current catalyst activation decision processing is initial (i.e., when the decision step S901 results in affirmation xe2x80x9cYESxe2x80x9d), a catalyst activation flag and the counter value CNT are cleared to xe2x80x9c0xe2x80x9d (zero) in a step S902, whereon the processing proceeds to a step S903.
On the other hand, when the decision step 901 results in that the current catalyst activation decision processing is not initial (i.e., when the decision step S901 results in negation xe2x80x9cNOxe2x80x9d), the processing proceeds straightforwardly to the step S903.
In the step S903, the operation state signals of the engine 1 (i.e., signals indicative of the current engine state) are fetched, as described previously, and decision is made as to whether or not the engine operation state signals indicate rise of the temperature Tc of the catalyst (hereinafter also referred to as the catalyst temperature) in a step S904.
By way of example, when the intake air quantity Qa of the engine 1 attains or exceeds a predetermined intake air quantity Qa1, as illustrated in FIG. 12, it is decided that the catalyst temperature Tc has risen (i.e., xe2x80x9cYESxe2x80x9d). In this way, whenever the engine operation state indicates the rise of the catalyst temperature Tc, the counter value CNT is incremented by one (step S905).
Subsequently, the current counter value CNT is compared with a maximum counter value CNTmax to thereby decide whether or not the current counter value CNT is equal to or greater than the maximum counter value CNTmax (i.e., CNTxe2x89xa7CNTmax) in a step S906. In this conjunction, it should be mentioned that the maximum counter value CNTmax may be so set as to correspond to the catalyst temperature Tc risen up to the activation level Tc1 at which the catalyst is activated sufficiently.
When it is decided in the step S906 that the current counter value CNT is equal to or greater than the maximum counter value CNTmax (i.e., when the decision step S906 results in xe2x80x9cYESxe2x80x9d), the counter value CNT is held at the maximum counter value CNTmax with the catalyst activation flag being set to xe2x80x9c1xe2x80x9d (step S907).
By contrast, when it is decided in the step S906 that the current counter value CNT is smaller than the maximum counter value CNTmax (i.e., when the decision step S906 results in xe2x80x9cNOxe2x80x9d), the catalyst activation decision processing is terminated without executing any further processing.
At this juncture, it should be mentioned that the state where the counter value CNT is smaller than the maximum counter value CNTmax means that although the engine is in the operation state in which the catalyst temperature T increases (see period ta in FIG. 12), the catalyst 10 is not heated up to the activation temperature Tc1 and thus the counter value CNT is yet short of the maximum counter value CNTmax.
By contrast, when the counter value CNT is equal to or exceeds the maximum counter value CNTmax, this means that the catalyst temperature Tc becomes equal to or higher than the activation temperature Tc1 and that the catalyst 10 is in the state prevailing after an activation time point t1, as is illustrated in FIG. 12.
Accordingly, when the current counter value CNT becomes equal to or greater than the maximum counter value CNTmax, the catalyst activation flag is set to xe2x80x9c1xe2x80x9d for thereby indicating that the catalyst 10 is in the activated state, and the counter value CNT is maintained at the maximum counter value CNTmax.
On the other hand, when it is decided in the step S904 that the engine is not in the operation state in which the catalyst temperature Tc can increase (i.e., when the decision step S904 results in xe2x80x9cNOxe2x80x9d), the counter value CNT is decremented by one (step 908), whereupon decision is made as to whether the counter value CNT as decremented is xe2x80x9c0xe2x80x9d (zero) or not (step S909).
When it is decided that the current counter value CNT is not greater than xe2x80x9c0xe2x80x9d or CNT≯0 (i.e., when the decision step S909 results in affirmation xe2x80x9cYESxe2x80x9d), the catalyst activation flag is reset to xe2x80x9c0xe2x80x9d (step S910), whereupon the catalyst activation decision processing illustrated in FIG. 11 comes to an end.
By contrast, when it is decided that the current counter value CNT is greater than xe2x80x9c0xe2x80x9d (i.e., when the decision step S909 results in negation xe2x80x9cNOxe2x80x9d), the catalyst activation decision processing is terminated straightforwardly.
At this juncture, it should be mentioned that the engine operation state in which the catalyst temperature Tc can not rise corresponds to the state in which the intake air quantity Qa is smaller than the predetermined intake air quantity Qa1, as can be seen from FIG. 12.
Further, the state in which the counter value CNT is greater than xe2x80x9c0xe2x80x9d means that although the catalyst 10 is in the activated state with the catalyst temperature Tc being higher than the activation temperature Tc1, the catalyst temperature Tc is lowering (period tb) even though the catalyst remains continuously to be in the activated state (see the state during a period to illustrated in FIG. 12).
Accordingly, when the counter value CNT is greater than xe2x80x9c0xe2x80x9d (zero), the catalyst activation flag is held at xe2x80x9c1xe2x80x9d, whereon the catalyst activation decision processing is terminated.
Furthermore, in the case where the counter value CNT is not greater than xe2x80x9c0xe2x80x9d (zero) (i.e., when CNT≯0), indicating that the engine is not in operation state to increase the catalyst temperature, this means that the catalyst 10 is in the state prevailing in precedence to the period to or alternatively in the state prevailing after the time point t2, as is illustrated in FIG. 12).
Thus, when the counter value CNT is not greater than xe2x80x9c0xe2x80x9d (zero), the counter value CNT and the catalyst activation flag are reset to xe2x80x9c0xe2x80x9d, to thereby indicate the state in which the catalyst 10 is not activated.
Next, by referring to a flow chart of FIG. 13, description will be made of the catalyst deterioration decision processing procedure by the conventional apparatus.
Referring to FIG. 13, the catalyst deterioration decision means 102 makes decision on the basis of the engine operation state signals inputted to the ECU 100 as to whether or not the operation state of the engine 1 is a predetermined operation state (step S101).
At this juncture, it is contemplated with the phrase xe2x80x9cpredetermined engine operation statexe2x80x9d to mean the state which is suited for the decision as to deterioration of the catalyst 10, i.e., a steady or cruising state of the engine or motor vehicle exclusive of the idling state and the accelerating/decelerating state.
When it is decided that the engine 1 is not in the predetermined engine operation state (i.e., when the decision step S101 results in negation xe2x80x9cNOxe2x80x9d), the catalyst deterioration decision processing illustrated in FIG. 13 is terminated straightforwardly.
By contrast, when it is found that the engine 1 is in the predetermined engine operation state (i.e., when the decision step S101 results in affirmation xe2x80x9cYESxe2x80x9d), then it is decided whether or not the catalyst activation flag is xe2x80x9c1xe2x80x9d (step S102).
When the catalyst activation flag is not xe2x80x9c1xe2x80x9d, (i.e., when the decision step S102 results in xe2x80x9cNOxe2x80x9d), the catalyst deterioration decision processing is terminated without executing any further processing.
On the other hand, when it is decided that the catalyst activation flag is xe2x80x9c1xe2x80x9d (i.e., when answer of the decision step S102 is xe2x80x9cYESxe2x80x9d), this means that the catalyst 10 has been activated. Accordingly, the air-fuel ratio signals V1 and V2 are fetched for executing the decision as to the state of activation of the catalyst 10 (step S103), whereon a catalyst deterioration decision parameter CHK is determined arithmetically (step S104).
The catalyst deterioration decision parameter CHK may be determined in terms of a ratio of variation of the second air-fuel ratio signal V2 relative to that of the first air-fuel ratio signal V1. In general, so long as the catalyst 10 is normal, the variation or change of the second air-fuel ratio signal V2 remains small over the activation period to, as is indicated by a solid line curve in FIG. 12. However, when the catalyst 10 is abnormal, remarkable variation of the second air-fuel ratio signal V2 makes appearance during the activation period to, as is indicated by a broken line curve in FIG. 12.
Subsequently, decision is made whether or not the catalyst deterioration decision parameter CHK is greater than a predetermined value (step S105). When it is decided that the catalyst deterioration decision parameter CHK is greater than the predetermined value (i.e., when the decision step S105 results in xe2x80x9cYESxe2x80x9d), this means that the second air-fuel ratio signal V2 varies or changes with large magnitude of voltage at a relatively high frequency. Accordingly, it is determined that the catalyst 10 suffers deterioration (step S106).
In that case, the alarm means 19 is activated (step S107) to inform the driver of deterioration of the catalyst 10, whereupon the catalyst deterioration decision processing comes to an end.
By contrast, when it is found in the step S105 that the catalyst deterioration decision parameter CHK is not greater than the predetermined value (i.e., when the decision step S105 results in xe2x80x9cNOxe2x80x9d), this means that change of the second air-fuel ratio signal V2 is insignificant. Thus, it is determined that the catalyst 10 is normal (step S108).
In succession to the step S108, the alarm lamp 19 is opened (step S109) if the alarm lamp 19 is lit. Otherwise, the catalyst deterioration decision processing is terminated straightforwardly.
As is apparent from the foregoing description, with the conventional apparatus, decision as to deterioration of the catalyst 10 can be performed only in the state where the catalyst 10 has been activated.
However, the state in which the catalyst temperature Tc is rising or lowering can not definitely be determined only on the basis of the condition that the intake air quantity Qa is equal to or greater than the predetermined intake air quantity Qa1 or that the former is smaller than the latter, because the catalyst temperature Tc is affected by the actual value itself of the intake air quantity Qa as well.
Accordingly, it is impossible to determine positively the activation state of the catalyst 10 with high accuracy and reliability on the basis of only the conditions illustrated in FIGS. 11 and 12.
As is apparent from the foregoing description, in the conventional catalyst deterioration detecting apparatus for the internal combustion engine, decision as to deterioration of the catalyst 10 is performed on the presumption that the catalyst 10 is activated when the predetermined engine operation state in which the intake air quantity Qa is not smaller than the predetermined intake air quantity Qa1 has continues for a predetermined period ta. Consequently, there arises a problem that the actual catalyst temperature Tc can not be detected because of inadequacy of the conditions for incrementation/decrementation of the counter value CNT.
In other words, the conventional catalyst deterioration detecting apparatus suffers a problem that deterioration of the catalyst 10 can not be detected with high reliability because the activated state of the catalyst 10 can not be determined with high accuracy.
In the light of the state of the art described above, it is an object of the present invention to provide an improved catalyst deterioration detecting apparatus for an internal combustion engine which can ensure an enhanced reliability for the decision as to deterioration of the catalyst by using a parameter value(s) indicating the engine load(s) and reflected in the counter value, for thereby increasing the degree of freedom concerning the conditions for incrementation/decrementation of the counter value so that various dispersions as encountered in the processing(s) can be substantially canceled out for while increasing the chance of the catalyst deterioration decision by detecting the temperature rise state of the catalyst essentially without fail.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a catalyst deterioration detecting apparatus for an internal combustion engine equipped with a catalytic converter for purifying exhaust gas thereof, which apparatus includes an engine load detecting means for determining arithmetically parameter values corresponding to load states of an internal combustion engine, an accumulating means for determining arithmetically an accumulated value by adding accumulatively counter values corresponding to the parameter values, a first comparison means for comparing the accumulated value with a first predetermined value corresponding to an operative temperature of a catalytic converter, and a catalyst deterioration decision means for making decision as to deterioration of the catalytic converter when the accumulated value attains or exceeds the first predetermined value.
By virtue of the arrangement mentioned above, the parameter value indicative of the engine load state can be reflected in the counter value, whereby an enhanced reliability can be ensured for the catalyst deterioration decision because the temperature rise state of the catalyst can be detected with high accuracy.
In a preferred mode for carrying out the invention, the catalyst deterioration detecting apparatus may further include a second comparison means for comparing the parameter value with a second predetermined value relevant to the temperature rise state of the catalyst. In that case, the accumulating means can be so designed as to determine arithmetically the accumulated value when the parameter value attains or exceeds the second predetermined value. With this arrangement, reliability of the catalyst deterioration decision can further be enhanced.
In another preferred mode for carrying out the invention, the engine load detecting means may be so designed as to arithmetically determine the intake air quantity of the engine as the parameter value. In this case, reliability of the catalyst deterioration decision can equally be enhanced.
In yet another preferred mode for carrying out the invention, the engine load detecting means may be so arranged as to arithmetically determine as the parameter value a negative pressure which is prevailing within an intake pipe of the engine. Similar advantageous effects as mentioned above can be achieved.
In still another preferred mode for carrying out the invention, the engine load detecting means may be so designed as to determine arithmetically a throttle opening degree as the aforementioned parameter value.
Owing to the arrangements described above, the catalyst deterioration detecting apparatus can reflect correctly and accurately the parameter values indicative of the engine loads in the counter values, whereby enhanced reliability can be ensured for the catalyst deterioration decision because of capability of detecting the operative state of the catalyst positively and reliably.
In a further preferred mode for carrying out the invention, the accumulating means may be so designed as to incorporate a data table or map for determining arithmetically the counter value(s) corresponding to the parameter value(s).
In a yet further preferred mode for carrying out the invention, the catalyst deterioration detecting apparatus may additionally be provided with a third comparison means for comparing the parameter value with a third predetermined value relevant to the temperature fall state of the catalyst. In that case, the accumulating means can be so designed as to subtract a counter value corresponding to the parameter value from the accumulated value if the parameter value is not greater than the third predetermined value.
In a still further preferred mode for carrying out the invention, the third predetermined value may be set at a value smaller than the second predetermined value, and the accumulating means may be so arranged as to hold the accumulated value as it is when the parameter value assumes a value falling between the second predetermined value and the third predetermined value.
In yet another preferred mode for carrying out the invention, in the case where the parameter value is not greater than the third predetermined value, the accumulating means may be so designed as to set a subtraction-destined counter value to be variable in dependence on the accumulated value at the time point the parameter value is detected.
In still another preferred mode for carrying out the invention, the accumulating means may be so implemented as to set the subtraction-destined counter value to be smaller as the accumulated value at the time point of detection of the parameter value is increasing.
In a further preferred mode for carrying out the invention, the catalyst deterioration detecting apparatus may additionally include an engine operation state decision means for deciding whether or not the operation state of the engine falls within a predetermined operation range. In that case, the accumulating means can be so designed as to suspend the arithmetic operation for determining the accumulated value if the operation state of the engine falls within the predetermined operation range.
In another preferred mode for carrying out the invention, the engine operation state decision means may be so implemented as to determine that the engine operation state falls within the predetermined operation range to thereby maintain the accumulated value, in case the engine is operating in an enrich mode.
In a further preferred mode for carrying out the invention, the engine operation state decision means may be so designed as to determine that the engine operation state falls within the predetermined operation range when the engine is operating in other modes than the air-fuel ratio feedback control mode, to thereby hold the accumulated value as it is.
In a still further preferred mode for carrying out the invention, the catalyst deterioration detecting apparatus may be additionally provided with a fuel-cut mode decision means for deciding whether or not the engine is operating in the fuel-cut mode. In that case, the accumulating means can be so designed as to execute the arithmetic operation for subtracting the accumulated value if the engine is operating in the fuel-cut mode.
With the arrangements described above, there can be realized the improved catalyst deterioration detecting apparatuses for the internal combustion engine which can ensure the enhanced reliability for the decision as to deterioration of the catalyst by using the parameter value(s) indicating the engine load(s) and reflected in the counter value(s) while increasing the degree of freedom concerning the conditions for incrementation/decrementation of the counter value so that various variable factors as encountered in the processing can be compensated for and that the frequency of the chance of the catalyst deterioration decision performed by detecting the temperature rise state of the catalyst can be increased. Thus, significantly advantageous effects can be achieved by the present invention.